[member="Tellos"]
Prior to 3000 BBY the galaxy was unmapped beyond the Outer Rim (hence Unknown Regions, Wild space, etc). While passing by planets during hyperspace travel you will miss every single species that lives on every single planet you don't stop to look at. Korriban and Ziost and such were beyond the outskirts of mapped known space, it wasn't until Naga Sadow attacked Empress Teta that it became a known region in space - known then as "Sith Space". Dromund Kaas was even hidden through various means by the Sith Emperor. The Sorcerers of Tund were known but were forgotten after Skere Kaan and the Sith Brotherhood began conquering the entirety of the known galaxy, culminating in Bane's thought-bomb that ended that conflict, and Tund faded into obscurity.
The Great Hyperspace Wars was when the region was discovered, but the Galactic War was the end of their species due to Darth Vitiate/Sith Emperor's galaxy-sprawling Empire being destroyed and then hunted down by the Jedi following that, which led to the Sith Brotherhood as mentioned above.